title: Ultrastructure of the myocardium of the least shrew, Cryptotis parva Say.
abstract: The heart of the least shrew, Cryptotis parva Say, is an extremely active organ, capable of achieving rates of 800-1,200 beats/minute. The general features of myocardial cell ultrastructure in this insectivore are much like those of other small mammals; no single striking feature of fine structure is present to which the physiological properties of this heart might necessarily be attributed. Still there exist in these myocardial cells a number of atypical properties. These include 1) mitochondria having a wide variety of sizes and internal configurations 2) a pleiomorphic, highly ramified, small-diameter transverse-axial tubular system (TATS) 3) numerous "labyrinths," which are proliferated components of the TATS, and 4) myofibril-free regions, located both in juxtanuclear and other myoplasmic levels and populated by a concentration of TATS elements and fibrillar structures. Features (2) and (3) are also characteristic of another fast-beating heart, that of the mouse. The sinoatrial and atrioventricular nodal regions, as well as a Purkinje system, have been identified in the least shrew heart, along with sparsely distributed atrial cells whose myofibrils contain proliferated Z-band material. A feature frequently encountered in atrial working muscle cells is the occurrence of close appositions between gap junctions and tubules of sarcoplasmic reticulum; such appositions are also present in other regions of the shrew heart, as are complexes composed of gap junctions and mitochondria.
authors.affiliation: Department of Physiology, University of Virginia School of Medicine, Charlottesville 22908.
authors.forename: M S
authors.initials: MS
authors.lastname: Forbes
authors.forename: O B
authors.initials: OB
authors.lastname: Mock
authors.forename: E E
authors.initials: EE
authors.lastname: Van Niel
mesh_terms: D000818:Animals; D005260:Female; D005614:Freeze Fracturing; D008297:Male; D008854:Microscopy, Electron; D008929:Mitochondria, Heart / Q000648:ultrastructure; D009206:Myocardium / Q000648:ultrastructure*; D012788:Shrews / Q000033:anatomy & histology*